The present invention concerns a procedure for detecting nonuniformities in the wall thickness of inaccessible metal pipes as well as a device particularly for carrying out the procedure.
Nowadays pipes, and in particularly pipelines that are buried or not directly accessible for other reasons are tested with so-called pigs or rabbits. Conventional pigs are known from: Non-destructive Testing Handbook, Second Edition, Volume 10, Non-destructive Testing Overview, American Society for Non-destructive Testing, 1996, p. 252. Here an essential testing objective is that of measuring the current wall thickness as a function of peripheral angle and length coordinate. To this end the pig is equipped with different probes, for instance for ultrasonic tests, leakage flux tests and the mechanical probing or measuring of the inside diameter.
Among the disadvantages or limits in the use of such pigs one finds pipe bends with overly small bending radius, nonaxial end joints with an overly high difference angle between the axes, an overly small pipe inside diameter, excessive changes in diameter, the off-line evaluation and a considerable technical effort.
DE 4004170 A1 concerns a procedure and device for the monitoring of appearance and propagation of fractures or crevices in materials, parts, components, buildings or celestial bodies, particularly in areas with seismic risks or during rock movements in tunnel construction. However, this procedure only provides for a determination of electric field variations. An active measuring arrangement in which a current is passed through pipes is missing in this document.
DE 4105842 A1 concerns a procedure for locating couplings in burried ferromagnetic pipelines. Here the detection of seals or couplings is the prime consideration. The procedure provides for an artificial magnetization of the pipe element via a field winding in the pipe. The passage of a current is not disclosed.
DE 3623588 A1 concerns a procedure for locating electric lines that are not in the open. This procedure provides for connection of the feed cables to an electric audiofrequency generator while a searching probe serves as an electrical audiofrequency receiver. Here again no anomalies in pipes are detected by passing currents.
Finally a procedure is known from U.S. Pat. No. 4,048,558 where a current is passed through a metal pipe with different frequencies, and the impedance is monitored.
The invention is based on the objective of indicating a procedure and a device for detecting nonuniformities in the wall thickness of inaccessible metallic pipes wherein it is possible to recognize without making contact, and from a certain distance, whether the wall thickness of a pipe to be surveyed is nonuniform peripherally. Such nonuniformities can arise as a consequence of corrosion, for instance from the inside. Pipes deeply buried and obscured by other layers should in particular also be made accessible to testing.
This objective is attained according to the invention by a testing procedure according to claim 1 and by a device particularly for carrying out the testing procedure according to claim 13. The invention provides a nondestructive testing procedure as well as a corresponding device for recognizing without contact nonuniformities in the wall thickness of inaccessible metallic pipes. Such nonuniformities being primarily produced by corrosion, this procedure is particularly useful for an early detection of wall thickness reductions due to corrosion in pipelines. The procedure is based on the passage of electric currents of different frequencies through the pipe. The depth of penetration of the current into the wall being a function of frequency, a shift in the center of gravity of the currents and thus in the magnetic field strength measured at the measuring distance from the pipe will result when damage is present. The device is transportable.
It is preferred that the alternating currents comprise two different frequency components. It is preferred that the magnetic fields are measured at the same place. It is further preferred that the two frequency components are generated by currents having the same amplitudes, and that the ratio of the magnetic field components is evaluated. It is further preferred to first calculate the ratios of magnetic field to current at the individual frequencies, then calculate their ratio and subsequently evaluate it. Then it is preferred that the currents having different frequencies are simultaneously passed through the pipe. It is further preferred that the return current is passed through a line which runs preferably parallel at a certain distance from the pipe being surveyed. Preferably, furthermore, a magnetic field probe is provided which measures the peripheral component of the magnetic field. Alternatively, a magnetic field probe is provided which measures the magnetic field component tangential to ground level. Alternatively, a magnetic field probe is finally provided which measures the magnetic field component normal to ground level.
Finally, preferably at least two probes are provided with which measurements are made simultaneously in order to be able to locate the pipe. The current having the lowest frequency can in the limiting case essentially be a direct current.
In the device particularly for carrying out the procedure, preferably current measuring equipment is provided which measures the current flowing through the pipe. It is furthermore preferred that the equipment for evaluation accepts as well the signal for the current supplied by the current measuring equipment, and evaluates the ratio of magnetic field signal to current signal.
Further advantages, features and applications of the present invention will result from the following description of preferred embodiments in connection with the drawing.